What is a Gamma Ray Air Shower?

Gamma Ray Air Showers
When a gamma ray hits the upper atmosphere it caninteract with the nuclei of an atom and create an electron/positron pair. Thisconverts all of the gamma ray's energy into the rest masses and kineticenergies of the particles and the gamma ray ceases to exist. The rest mass ofan electron (0.5 MeV/c2) is tiny compared to the energy carried bythe gamma ray, so the rest of the energy is converted to kinetic energy. Meaning that theelectron and positron are both created traveling at virtually the speed oflight. These particles then emit more gamma rays as they pass through theatmosphere. These secondary gamma rays have enough energy to create moreelectron/positron pairs. This cascade continues until all of the energy hasbeen parceled out among tens of thousands of particles all traveling towardsthe groung at close to the speed of light. These electrons and positrons giveoff a characteristic blue flash via the Cerenkoveffect which is used to trigger ground based telescopes.

Monte Carlo Simulations of Air Showers
Through a series of programs developed by Kertzman and Sembroski, we are ableto model gamma ray air showers. In the KASCADE programs, we enter inputs suchas energy, observation altitude, and shower direction into the computer. Asthe simulations go on, data files are written out which contain thecharacteristics of the gamma ray shower. These data files are then convertedinto a form which is able to be used in several different software packages.Because we know the values which areinput into the simulation, we can use the output data to experiment withdetection methods and techniques for distinguishing between gamma ray andcosmic ray induced showers. The KASCADE programs follow the electrons andpositrons of the air shower, keeping track of their energy, direction, andother pieces of information. This data is then read into another program(KASLITE) which generates the photons which would be produced by theseelectrons and positrons. For a typical 1 TeV shower over 60,000,000 photonsare produced and kept track of.
A Simulated Gamma Ray Shower

Photons on the Ground From a 50 GeV Shower at VERITAS
Wavefront of 50 GeV Shower at VERITAS
This image is from a simulation. The green dots represent photons generated byelectrons and the red dots represent photons generated by positrons. The point0,0 is at the center and both the x and y-axis extend from -250 m to 250 m. The dense ring of photons occurs at 120 m.
This image is also from a simulation. The different colored dots correspond tothe different altitudes from which these photons were emitted. Blackcorresponds to an altitude of less than 6000 m, light blue to between 6000 and10,000 m, and purple to above 10,000 m.

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